The Applied Voltage Test (AV) is a fundamental dielectric test performed on power transformers to verify the integrity of insulation between windings and earth. Unlike impulse tests that simulate lightning or switching surges, the Applied Voltage Test checks the transformer’s ability to withstand power-frequency overvoltages under controlled conditions.
Defined in IEC 60076-3, the Applied Voltage Test is a routine test for all power transformers and is essential for confirming that the transformer insulation is free from major defects before it is energized in service.
This article explains the purpose, test procedure, test voltage selection, acceptance criteria, and engineering significance of the Applied Voltage Test for engineers, inspectors, and students.
Table of Contents
What Is the Applied Voltage Test (AV)?
The Applied Voltage Test is a power-frequency dielectric test in which a high AC voltage is applied between a transformer winding and earth (tank and core), while all other windings are grounded.
The test verifies the insulation system’s ability to withstand:
- earth faults
- temporary overvoltages
- system imbalance conditions
In IEC terminology, the test is sometimes called:
- Applied Voltage Withstand Test
- AC Withstand Test
- Line Terminal AC Test (LTAC) in certain configurations
Purpose of the Applied Voltage Test
The main purpose of the Applied Voltage Test is to confirm the dielectric strength of the main insulation to earth.
Specifically, the test verifies:
- winding-to-core insulation
- winding-to-tank insulation
- insulation of leads and connections
- clearances in oil and solid insulation
IEC 60076-3 includes this test to detect gross insulation defects such as:
- damaged paper insulation
- incorrect clearances
- contamination in oil
- assembly errors
- moisture-related weaknesses
Unlike impulse tests, the Applied Voltage Test stresses the insulation uniformly and continuously for a defined time.
When Is the Applied Voltage Test Required?
According to IEC 60076-3, the Applied Voltage Test is:
- A routine test for all power transformers, regardless of voltage class
- Performed on every manufactured unit
- Conducted after assembly and before delivery
There are no exemptions based on voltage level—distribution, medium-voltage, and extra-high-voltage transformers must all pass the Applied Voltage Test.
Applied Voltage Test vs Induced Voltage Test
It is important not to confuse the Applied Voltage Test with the Induced Voltage Test:
| Test | Main Purpose | Stresses |
|---|---|---|
| Applied Voltage Test (AV) | Insulation to earth | Winding-to-earth |
| Induced Voltage Test (IV) | Inter-turn insulation | Turn-to-turn, phase-to-phase |
Both tests are mandatory and complement each other.
Test Setup for the Applied Voltage Test
The Applied Voltage Test is performed using the following configuration:
- The winding under test is energized with AC voltage
- All other windings are solidly grounded
- The transformer tank and core are grounded
- The test voltage is applied between the winding and earth
Only one winding is tested at a time, unless otherwise agreed.
Applied Voltage Test Voltage Levels
IEC 60076-3 specifies the required test voltages in its insulation coordination table. The Applied Voltage Test voltage depends on the Highest Voltage for Equipment (Um) of the winding.
Examples:
- Um = 36 kV → Applied voltage = 70 kV
- Um = 72.5 kV → Applied voltage = 140 kV
- Um = 145 kV → Applied voltage = 275 kV
Important rule
The applied test voltage must not be lower than the minimum value specified by IEC.
The voltage is measured as r.m.s. value and applied at rated frequency (50 or 60 Hz).
Test Procedure (Step by Step)
The Applied Voltage Test follows a clearly defined sequence:
1. Test Preparation
- Verify grounding of tank, core, and non-tested windings
- Check oil level and temperature
- Ensure measuring instruments are calibrated
2. Voltage Application
- Increase the AC voltage smoothly from zero
- Reach the full test voltage without overshoot
3. Test Duration
- Maintain the test voltage for 60 seconds
During this time, the transformer must withstand the voltage without breakdown.
4. Voltage Reduction
- Reduce voltage smoothly back to zero
- Avoid sudden disconnection
Acceptance Criteria for the Applied Voltage Test
A transformer passes the Applied Voltage Test when:
- No internal flashover occurs
- No external flashover occurs
- No breakdown of insulation is detected
- No sudden increase in current is observed
- No audible discharge or abnormal noise is present
IEC defines the test as withstand only—partial discharge measurement is not required during the standard Applied Voltage Test.
What Constitutes a Failure?
The test is considered failed if any of the following occur:
- Insulation breakdown
- Flashover inside the transformer
- Flashover on bushings (unless explicitly allowed)
- Protective device operation
- Sudden collapse of voltage
If failure occurs, IEC requires:
- investigation of the cause
- repair of the transformer
- repetition of the test
Why the Applied Voltage Test Is Technically Important
The Applied Voltage Test simulates real operating stresses such as:
- earth faults
- temporary system overvoltages
- neutral displacement
- phase imbalance
These conditions can last seconds, not microseconds, making AC withstand testing critical for long-term insulation reliability.
A transformer that passes impulse tests but fails the Applied Voltage Test is not safe for service.
Special Considerations
Line Terminal AC Test (LTAC)
In some cases, IEC allows applying the AC voltage only to the line terminal instead of the full winding. This is specified separately and must be agreed with the purchaser.
High-Voltage Transformers
For EHV transformers, clearances in air and oil must be sufficient to avoid external flashover during the test.
Bushings
Bushing insulation must be suitable for the applied voltage; otherwise, protective measures may be required.
Applied Voltage Test in Transformer Quality Control
The Applied Voltage Test is one of the final acceptance tests before shipment. It provides confidence that:
- the insulation system is correctly designed
- manufacturing quality is consistent
- assembly defects are not present
Because it is performed at power frequency and full stress duration, it is one of the most reliable indicators of basic insulation integrity.
Simple Summary
The Applied Voltage Test (AV):
- is a routine IEC 60076-3 test
- verifies insulation between winding and earth
- uses AC voltage at rated frequency
- is applied for 60 seconds
- must meet IEC minimum voltage levels
- must show no breakdown or flashover
It is an essential step in ensuring transformer safety and long-term performance.
Conclusion
The Applied Voltage Test plays a critical role in transformer dielectric testing. By applying a controlled power-frequency voltage between the winding and earth, IEC 60076-3 ensures that transformers can withstand real system overvoltages without insulation failure. Together with impulse and induced voltage tests, the Applied Voltage Test forms a complete dielectric verification process that protects utilities, industries, and power networks from costly transformer failures.